The invention generally relates to a radiation detecting device and particularly to an improved device for detecting alpha particles in garments used in nuclear power or nuclear material handling facilities.
Unlike gamma and beta type radiation alpha particle radiation is a type of non-penetrating radiation which is easily blocked by thin amounts of a fabric or even air. Consequently, a radiation detector must be placed closely adjacent to an article being examined to detect alpha radiation contamination. In the past, garments suspected to being contaminated have been frisked with small hand-held probes. While hand frisking allows the operator to hold the probe close enough to detect alpha particles, it is time consuming, and contaminated areas of a garment may be overlooked as a result of operator error. In addition, special training is required for the operator to be able to monitor very low radiation count rates.
Conveyer-type devices for simultaneously detecting gamma, beta and alpha radiation are know. A conveyer apparatus for detecting radioactive material in garments is disclosed in applicant's copending application Ser. No. 07/201,807, filed June 2, 1988, which is incorporated herein by reference. Generally, these devices include a horizontal conveyer belt with a radiation detector mounted adjacent to the belt for determining whether any of the garments radiate unacceptable levels of radioactivity. Such devices are typically used after the garments have been subjected to a decontamination cleaning process. The garments are then fed into the conveyer detection device. If the level of radiation detected exceeds a predetermined level, an alarm notifies the operator that further decontamination or disposal is necessary. Unfortunately applicants have observed that conventional automated monitoring systems, do not insure that all areas of the article being scanned will be positioned sufficiently close to the detector to allow detection of short-range alpha particle radiation. In addition, the conveyer belts will often shield (by blocking or increasing the particle travel distance through air) the radiation from the detectors.
Another problem with known conveyer devices that applicants have observed is that static charges may develop as a result of garments rubbing together during drying, or as a result of friction between the conveyer belts or other parts of the monitoring system. Static charges discharging into or around the detector may damage the detector or cause electrical noise which may be incorrectly interpreted as radiation. Static charges can also deflect or distort the path of alpha particles, thereby preventing them from reaching the detector. In addition, static charges attract dust or other foreign matter to the conveyer belt which may block radiation from detection. Naturally occurring radon gas daughter products are also attracted by static charges, causing false alarms due to the detection of the radiation emitted by these substances.
A further problem with conventional radiation detecting systems that applicants have observed stems from the use of the same detectors to detect both alpha and beta particles. Specifically, in the gas-flow proportional detectors used in such systems, a single voltage is utilized for the detection of both the alpha and beta particles. However, the optimum voltage for detecting alpha particles is different from that for detecting beta particles, and thus the voltage utilized for detecting both is a compromise. Such a compromise voltage results in a background noise which is too high for accurately measuring low levels of alpha contamination, while being too low for adequate sensitivity to beta radiation.
Clearly, a conveyer-type radiation detecting device is needed which is capable of reliably detecting low energy radiation such as alpha particles in garments as they are conveyed. The device should provide detectors located sufficiently close to every section of the garment as it travels such that all areas of the garment can be accurately monitored for alpha radiation. In addition, the device should be capable of minimizing the possibility of erroneous readings resulting from contaminants on the system. The device should also allow the detectors to be operated under conditions optimized for detecting alpha radiation.